Due to the adverse toxicological properties of corrosion inhibiting pigments based on heavy metals, the industry is searching for environmentally acceptable alternatives. ICPs, such as polyaniline provide such an alternative. ICPs are believed to provide corrosion protection by stabilizing the potential of the metal in the passive region by forming and maintaining a protective oxide layer on the surface of the metal. A key advantage of ICPs is their ability to provide tolerance to pin holes and scratches, due to their conductivity and redox chemistry. Since the coating is electrically conductive, the entire coating acts to passivate any areas of exposed material. With conventional barrier coatings, pin holes and scratches are the main source of coating failure, which necessitates multiple coatings.
Anodic protection of steel with electrochemically deposited polyaniline has been known for over a decade, see for instance, D. W. DeBerry, J. Electrochem, Soc., 132, 1022-26 (1985). Several reports have appeared since then which describe the use for ICPs for corrosion protection of steel. Most of the earlier studies have utilized conductive coatings prepared by electrochemical polymerization directly on the surface of the steel, while other studies have utilized neat solutions of neutral polyaniline in 1-methyl-2-pyrrolidone. These reports disclose various approaches for preparing coatings/coated specimens for evaluating corrosion protection using ICPs.
Thompson et al., Los Alamos National Laboratory report LA-UR-92-360, reported on the use of corrosion protective coatings using ICPs. Mild steel samples were coated with a solution of emeraldine base of polyaniline in 1-methyl-2-pyrrolidone. After drying, the undoped polyaniline was doped to the conducting state. A top coat of crosslinked epoxy was employed to impact abrasion resistance. Significant corrosion resistance was claimed in 3.5% sodium chloride and 0.1 M hydrochloric acid. Such approaches lack industrial application however, due to the limited solubility & stability of polyaniline/NMP solutions.
B. Wessling, Adv. Mater., 6, 226-228 (1994), reported passivation of metals using dispersions of polyaniline. Polyaniline was deposited from pure polyaniline dispersions on metallic samples. The coating procedure was repeated 5 to 20 times to provide thicker coatings. A significant positive shift in corrosion potential along with reduction in corrosion current was reportedly observed. Upon removal of the polyaniline coating, a change in appearance was observed and the presence of passivated layer was confirmed. Multiple coatings are labor intensive and costly and thus not attractive on a industrial scale.
W. Lu, R. L. Eisenbaumer and B. Wessling, Synthetic Metals, 71, 2163-2166, (1995), reported corrosion protection of mild steel in acidic and saline atmosphere using neutral and doped polyaniline coatings, with a epoxy top coat. Neutral polyaniline was applied from NMP solutions, which were then further doped with p-toluene sulfonic acid. Both the neutral and doped polyaniline showed corrosion protection. Corrosion protection provided by doped polyaniline was more significant in acid conditions than saline conditions.
Sitaram et al have reported on corrosion protection of untreated steel using Versicon.RTM., a doped polyaniline, neutral polyaniline and PANDA, a soluble form of polyaniline manufactured by Monsanto. They reported PANDA exhibited significant improvement in corrosion protection, when used as a base coat with a conventional top coat. However, it was interesting to note that both Versicon.RTM. and PANDA did not exhibit significant protection, when formulated in to conventional coatings such as epoxy or acrylics. They concluded that polyaniline/PANDA do not function as a pigment.
Recently Miller et al., U.S. Pat. No. 5,648,416 have described corrosion resistant paints using non-conductive conjugated polymers; in particular, neutral polyaniline based alkyd and vinyl systems were evaluated. The coatings exhibited conductivity lower than 10.sup.-8 S/cm. The coatings provided improved corrosion protection using ASTM B-117 when compared to paints containing no polyaniline.
In summary, conductive polymers, specifically polyanillines have been shown to provide corrosion protection to steel. However, there is discrepancy as to the extent of corrosion protection and the effect of the form of the polyaniline and the nature of corrosion environment. The techniques used to coat the substrate to be protected include direct polymerization on the substrate, solution coating techniques or dispersions of ICPs. Sitaram et al., have found that blends of polyaniline with conventional resins are less effective than neat coatings.
Corrosion protective coatings based on intrinsically conductive polymers can have a wide range of commercial applications such as bridges, rebars used in concrete, underground storage tanks, ships, oceanic drilling platform equipment, automotive and several industrial machinery, equipment and metal furniture.
In order for polyaniline and other ICPs to be successful commercially in corrosion prevention, it is apparent that they need to be applied as coatings using practical techniques. Further, these coatings need to be environmentally attractive. In addition to processability of ICPs, the coatings must provide excellent adhesion to the substrate metal, and be durable and environmentally stable in certain applications. Thus, there is a clear need for ICP coatings that can provide enhanced corrosion protection which are environmentally compliant, possess good adhesion to steel, and abrasion resistance.
Electrochemical deposition techniques lack application to large and existing structures. Solutions of polyaniline in NMP have limited shelf life. ICP coatings made by dispersion of polyaniline such as those described in U.S. Pat. Nos. 5,494,609, and 5,648,416, can find application as corrosion inhibiting primers. However, the dispersion techniques are rather involved and may not be practical industrially. Further, they require polyaniline in the right morphology and composition for optimum dispersion. Due to high oil absorption of polyaniline (Versicon.RTM.), it is difficult to formulate high solids, low VOC (volatile organic content) coatings.
Polyaniline and other conducting polymers have been successfully coated on carbon black and other high surface area substrates to achieve higher conductivity. The conductive polymer is polymerized in the microporous substrate and provides high conductivity. The resulting product is black, due to carbon black, and its dispersibility in paint is unknown. Moreover, high surface area/high oil absorption of the conductive polymer restricts the loading and degrades the film properties of the paint. Further, the use and effectiveness of these materials for corrosion prevention is not known.